Looks like it is hardcoded somewhere, I don't suppose there is a way to change that.find the line:
<texture depth="2" name="mapViewTexture" width="712" height="712" pos="0,0" material="Materials/MaskableMainmap"/>
the map texture size is set there (712x712), change it - if there is no result, then it is still hardcoded.
I'm ok with editing .dll files. I never play on public servers anyway so no an issue for me to disable EAC.I think, i can tell you where it is in the .dll code ... but it's a .dll...